If I went with a HX on the spare Sanyo, it would be configured like the Daikin Altherma Bi-Valent setup..
Which is almost what I'm doing right now. Turning off the Sanyo below 5deg F
and turning on the oil heat (forced baseboard hot-water).
I just call it the Hybrid Mode.
Having the HydroBox outdoors might be a good idea, just in case the
government gets really crazy and outlaws R410A, and forces us to use R290..